The present invention relates to an air-fuel ratio control method for internal combustion engines, and more particularly to a method of properly controlling the air-fuel ratio of a mixture of fuel supplied to the engine after interruption of fuel supply which is effected when the engine is in a predetermined decelerating condition.
Conventionally, it is well known to control the air-fuel ratio of a mixture supplied to an internal combustion engine (hereinafter referred to as "the actual air-fuel ratio") by sensing the concentration of an ingredient of exhaust gases emitted from the engine by means of an exhaust-gas concentration sensor provided in the exhaust system of the engine, and effecting feedback control in response to the sensed ingredient concentration to bring the air-fuel ratio to a desired set value, to thereby improve the fuel consumption and emission characteristics, etc., of the engine.
It is also known to interrupt supply of fuel to an internal combustion engine when the engine is in a predetermined decelerating condition (hereinafter referred to as "fuel cut"), in order to improve the fuel consumption, etc. of the engine.
However, the feedback control of the air-fuel ratio had a difficulty when it is carried out to restart the fuel supply following the fuel cut.
Specifically, when the fuel cut is being carried out, the actual air-fuel ratio is extremely lean. When the fuel cut is terminated, the feedback control is resumed to control the air-fuel ratio in response to the extremely lean actual air-fuel ratio. Consequently, the actual air-fuel ratio becomes temporarily overrich due to control delay.
To solve the above problem, an air-fuel ratio control method has been proposed, e.g., by Japanese Patent Publication (Kokoku) No. 58-6052, which restarts the feedback control when a predetermined time period has elapsed after resumption of fuel supply to the engine following fuel cut, thereby coping with the control delay.
However, the proposed method has the disadvantage that the actual air-fuel ratio cannot be properly controlled to a desired set value immediately after termination of the fuel cut, which may lead to increased fuel consumption and degraded emission characteristics.
Specifically, when fuel cut is carried out, almost all the fuel adhering to the inner wall of the intake pipe is supplied to the engine cylinders. Accordingly, immediately after termination of the fuel cut, a considerable part of the fuel injected adheres to the intake pipe inner wall so that the actual air-fuel ratio is temporarily leaned. Thereafter, the actual air-fuel ratio enters a transient state wherein it gradually becomes closer to a desired air-fuel ratio with an increase in the amount of fuel adhering to the intake pipe inner wall, and then the actual air-fuel ratio shifts into a steady state wherein the amount of the adhering fuel no longer increases.
On the other hand, the proposed method uses an exhaust gas-ingredient concentration sensor of the type having an output invertible in response to a change in the actual air-fuel ratio across a desired air-fuel ratio having a fixed value, e.g., a stoichiometric ratio.
Therefore, even if the above predetermined time period is set to a relatively short value to advance the commencement timing of the feedback control, i.e., if the feedback control is commenced when the actual air-fuel ratio is still in the transient state, there exists a large difference in value between the actual air-fuel ratio which is still lean and the fixed desired air-fuel ratio so that the air-fuel ratio is largely controlled to compensate for the difference or enrich the mixture. As a result, the actual air-fuel ratio becomes overrich when it reaches the steady state, spoiling the controllability of the air-fuel ratio.
Conversely, to eliminate this disadvantage, if the predetermined time period is set to a relatively long value to retard the commencement timing of the feedback control, the feedback control is not carried out during the transient state, thereby lowering the accuracy of the air-fuel ratio control.